10 Classic Optical Illusions

Our perception of the world around us is limited by our senses, none of which gives us more information than the human eye. But our vision is often fallible, and can be fooled by natural and manmade optical illusions. Thousands of such illusions have either been discovered or created, but here are 10 notable optical illusions that demonstrate psychological tricks of the mind and eye. (Note: Some of these illusions are more noticeable on larger monitors than PDAs.)

10. The Moon Illusion

The Moon appears much larger when viewed just above the horizon; J.P. Stanley

Mankind has puzzled over this illusion since ancient times: why does the full moon appear larger on the horizon than when it is overhead? In fact, both are exactly the same size. This can easily be proven by taking a series of images of the rising Moon as it moves from the horizon toward the zenith. Many people mistakenly blame atmospheric distortion or foreground perspective as the cause. Doubtless the Ponzo Illusion (see No. 9 below) comes into play, although again, the illusion vanishes photographically. It should be noted that viewing the Moon upside down also causes the illusion to disappear. Some controversy still exists as to why this is so, and current thinking is that the human brain has trouble with large overhead spatial perspectives — known by scientists as the “sky bowl” illusion — and simply “expects” the Moon to appear larger on the horizon.

9. The Ponzo Illusion

The yellow lines are the same size; Photo credit: NASA Same Shade Illusion: Edward H. Adelson

First described by psychologist Mario Ponzo in 1913, the Ponzo illusion is a false impression of perspective. Although the two lines superimposed on the convergent ones appear to be of dissimilar lengths, they are in fact the same. Again, this demonstrates that much of what occurs between the brain interpreting what the eye sees is expectation. This illusion works either in a horizontal or vertical frame.

8. The Zöllner Illusion

The long black lines are actually parallel; Fibonacci

Noted by astrophysicist Carl Zöllner in 1860, this effect causes us to see expected movement amongst static lines. This is because the short and long alternating lines appear to be convergent, although they are in fact parallel. The effect is most apparent when you relax your eyes and take in the entire image. Curiously, the effect vanishes if depicted with green lines on a red background of equal brightness.

7. The “Same-Shade” Illusion

Squares A and B are the same color.

This illusion is so uncanny that some folks can’t believe this one even when conclusively shown the proof. Both blocks “A” and “B” are actually the same shade. Again, our brain is fooled by the checkerboard pattern versus the shadow cast by the cylinder. Part of the genius of this illusion is that it demonstrates the priority the brain puts on the geometry presented even over the true light contrast depicted.

6. The Müller-Lyer Illusion

The horizontal lines are all parallel; Fibonacci

A size consistency illusion, this exercise asks the viewer to guess at the approximate center point of the line depicted. Almost always, people will tend to underestimate the bisection point. Interestingly, there is a cultural component to this illusion, with younger, urbanized populations showing a higher susceptibility to the Müller-Lyer Illusion in studies, possibly because of expectations from living in a manufactured environment composed of clean, sharp edges.

5. The Autokinetic Effect

The pattern is stationary, although your eyes depict constant movement.

This is a wonderful example of perceiving motion where there is none. Staring at a fixed star long enough will also produce the same effect. Our hunter-gatherer stereovision evolved for sensing motion against a background, and often expects and inserts such motion when confronted with a lack of depth. Interestingly, you can make the effect at least partially disappear by focusing on a small part of the diagram.

4. The Purkinje Effect

The flower is the same color in all three frames; Dick Lyon/Lewis Collard

The light-sensitive cells inside our eyes are known as cones, which are used in bright light, and rods, used in low light. Changes of perception of color vary with brightness. Rods are terrible at perceiving color, which is why a scene under low-light conditions can look very black and white. As intensity fades, the Purkinje effect can also cause contrast and color to shift across the spectrum, giving red objects a greenish tinge.

3. Kanizsa Triangle

Your eye sees a white triangle, although none is present; Fibonacci

Another feat that our brain is great at is filling in the gaps. This gift of visual “interpretation” is what makes password challenges like CAPTCHA function; we can be forgiving of the thousands of different font styles while a computer eye may fail in this regard. This is especially evident in the Kanizsa Triangle illusion first noted by psychologist Gaetano Kanizsa. This ability to infer an illusory shape probably served us well in the wild to spot the shadow outlines of both predator and prey. In fact, there’s some thought that the advent of symbolic interpretation in the human eye-brain system may have led to written language. The more you study these types of illusions, the more you encounter them in everyday life.

2. Café Wall Illusion

The horizontal lines really are parallel; Photo credit: Joe Bekker

A variant of the Zöllner Illusion, the Café Wall or Kindergarten Illusion is an extreme form combining parallel (yes, they are parallel!) lines and staggered blocks. Not only does your eye perceive the lines as sloping, but it also perceives the blocks as moving. Contrast between the black and white blocks (which the eye perceives as gaps) comes into play, as the effect also disappears when different colors are introduced. The accompanying photograph is a building at the Melbourne Docklands in Australia. Note the rings in front of the building, which are used to show that the black lines are indeed parallel.

1. Hybrid Image

Stand a few feet from your computer, you see Marilyn Monroe; move closer … it’s not Marilyn.

Walk away from your computer screen a few steps, and you see Marilyn Monroe; move closer, and it’s not really her … but the visage of a famous scientist. This illusion (and several variants found around the Internet) works because our eyes tend to “grab” the low-resolution background from afar, while zeroing in on the sharp lines close up. This also shows the predilection the human brain has for picking out faces, often perceived in cumulus clouds, the “Mars face” and the famous Man in the Moon.

Want more? Michael Bach maintains an excellent site with hundreds of examples of optical illusions. It’s also worth checking out the works of M.C. Escher.

One more: Haidinger’s Brush

Photo credit: CCA-SA 3.0 Daniel P. B. Smith

Did you know that some humans, bees and out-of-work-superheroes share something in common; the ability to “see” polarized light. An elusive effect, Haidinger’s Brush resembles two interlocked “bowties” aligned along the direction of polarization in the center of the field of view. Staring at a white computer screen, a deep blue sky, or a sunlit piece of white paper may cause this bizarre effect to appear, although the brain tends to like to reject things like “floating paint brushes” as not conforming with a sane reality. Use of a polarizing filter or sunglasses may also enhance the chances of catching this wacky optical phenomenon.

David Dickinson is a backyard astronomer, science educator and retired military veteran. He lives in Hudson, Fla., with his wife, Myscha, and their dog, Maggie. He blogs about astronomy, science and science fiction at www.astroguyz.com.